Technical questions--need help

[rfgordon]

I hope the wisdom in the group can answer these questions. I've exhausted the knowledge of the older engineers at work (the ones who actually studied tubes back in the day), and can't find ready answers elswewhere.

1. Some local blues guys were test-gigging one of my amps last night, and I had occasion to stick my head into the back in the near total darkness of the bar. The two 6L6s had bluish-purplish halos inside the tube. It seemed to ringh the inside of the glass. The tubes are 1960 German (branded Realistic), maybe Seimens or Telefunken, if that matters. They're run cathode biased, sharing a 250 ohm resistor, 430v on the plate and 415 on the screen. Any ideas on this phenomenon?

2. A single-ended OT needs to be air gapped in order to avoid being magnetized. Thus, conventional wisdom holds that one cannot use half the primary of a PP OT as a single ended OT, even though many of the Weber designs do. My question: does this rule apply to using half the pri on a toroidal OT?

3. In the older Fender amps that ran cathode bias, as in the schematic below, there are no resistors on the pins of the 6L6s. But on mor modern designs (also an example posted) it is more common to see resistors on the grid/screen. Is the purpose of the resistors to help stabilize the amp? Is it there for tone? Are there pros/cons to wiring the different ways?

4. In older Fender designs the 6L6s shared a much smaller cathode resistor than in modern designs. I have used Fender values, but the tubes always red plate. My esperimenting seems to show that higher plate voltages require larger cathode resistors, other things being equal. Is this correct?

Thanks for your wisdom.

Also, the guys at Durty Nelly's pub last night were truly grooving on my amp. Charlie Pasterfield (of Charlie Pasterfield and the Believers) called it "a hoss of an amp." It truly is gratifying when people enjoy making their music thru our creations!

[Bob-I]

Let me try to answer a few.

The two 6L6s had bluish-purplish halos inside the tube. It seemed to ringh the inside of the glass.

Tubes don't actually have a vacuum in them. The oxygen is displaced with another inert gas. Often these gasses glow in odd colors. Ignore it. Bright RED is the only thing to worry about.

2.

No clue.

3. In the older Fender amps that ran cathode bias, as in the schematic below, there are no resistors on the pins of the 6L6s. But on mor modern designs (also an example posted) it is more common to see resistors on the grid/screen. Is the purpose of the resistors to help stabilize the amp? Is it there for tone? Are there pros/cons to wiring the different ways?

The grid stoppers work with miller capacitance to reduce RFI. My guess is that Leo learned this the hard way and added the grid stoppers later. The screen's? Not sure but I'd guess he just didn't feel the need to protect the screens since tubes were so cheep.

4. In older Fender designs the 6L6s shared a much smaller cathode resistor than in modern designs. I have used Fender values, but the tubes always red plate. My esperimenting seems to show that higher plate voltages require larger cathode resistors, other things being equal. Is this correct?

AC voltage was lower in the 50's. Couple that with the lower voltage xformers Fender used and you can use a smaller Rk.

[MarkB]

Let me try to answer a few.

The two 6L6s had bluish-purplish halos inside the tube. It seemed to ringh the inside of the glass.

Tubes don't actually have a vacuum in them. The oxygen is displaced with another inert gas. Often these gasses glow in odd colors. Ignore it. Bright RED is the only thing to worry about.

Actually, tubes do have a vacuum inside them - that's why they're called vacuum tubes, after all.


The blue glow usually comes from electrons striking the inside of the glass envelope.

[Tdale]

Well, I checked, and there is not a real vacuum in my vacuum cleaner..

Sometimes a name is just a name.

Here is something from wikipedia:

"Unabsorbed free air sometimes ionizes and becomes visible as a pink-purple glow discharge between the tube elements."

Seems like that the vacuum isn't perfect, in most tubes...

Tommy

[VacuumVoodoo]

Blue glow. As long as the glow is on inside surface of the glass bottle it's nothing to worry about. It's just residual gas geting ionised. Makes a very poor neon lamp. Glowing cloud around and/or inside the plates is bad.

Vacuum. It's not absolute vaccum like deep in space becuase it's not possible to achieve. But the air pressue inside a tube is extremely low. It is further reduced in manufacturing by burning off the getter. BTW the getter is the shiny metallic deposit on inside of the bottle. The round metal ring or bowl or whatever is only a container which held the getter material before it was vaporised. It has no influence on tube performance, it's not electrically active.

There are tubes filled with inert gas on purpose. They're called kenotrons and were used as high voltage (in kV range) rectifier. Mercury vapor was used in some.

Red plating with new tubes vs. NOS in cathode biased amp. New production tubes will often do that. Their plate vs. grid bias characterics unfortunately differ from old originals. Increasing the value of cathode resistor is the correct remedy.

Screen grid resistors are there to protect the screen grid by limiting power dissipation in them. You limit the plate dissipation with proper bias. Think of screen grid resistor as having analogous effect on screen grids. Simplistic explanation of course.

Using PP transformer in SE circuit. NO, you can not build a 30W SE amp with a 30W rated PP transformer. YES, you can build a <10W Se amp with it.

PP amp rated 30W will have a transformer with max primary winding DC current rating of ca 100mA.

You can use it in SE amp with idle (no signal) plate current less than 100mA. It will not be a 30W amp but rather about 7W is possible.

A 30W SE amp would need much higher plate current and therefore require a much larger transformer. That's where the air gap comes to the rescue.

Air gap is not an absolute requirement for SE transformers. It's a smart way to reduce size and cost of the transformer for same output power.

[rfgordon]

Guys,
Thanks very much for our answers. I suspected most of them, but couldn't really be sure.

Here's one more for y'all: I can in no way claim to have seen all the designs that exist, but I can't recall any that use a bypass cap on the cathode resistor for the PI. Is there an electrical reason for this, or just tradition?

[Bob-I]

Actually, tubes do have a vacuum inside them - that's why they're called vacuum tubes, after all.

Sorry Mark, but no they don't have a vacuum in them. They're called vacuum tubes because the early tube makers attempted to get the oxygen out of them. Edison himself failed at that so he started experimenting with inert gasses to displace the oxygen. Todays tubes have a partial vacuum with an inert gas to replace the air.

[VacuumVoodoo]

If by raplacing air with inert gas you mean that the gas pressure inside the tube is same as air pressure outside then you're wrong. The pressure inside a tube is really very very low. In the manufacturing process the air is pumped out. How low pressure (or how high vacuum) is achieved is dependent on how good the vacuum pump is. After sealing the tube is inductively heated and getter burns off all remaining oxygen. All that is left are residual quantities of inert gases that were in the air. Inert gases are not pumped into the tube unless it's a kenotron or a neon light the color of which is defined by the gas used.

[Bob-I]

The pressure inside a tube is really very very low. In the manufacturing process the air is pumped out.

That's why I said partial vacuum

[MarkB]

Actually, tubes do have a vacuum inside them - that's why they're called vacuum tubes, after all.

Sorry Mark, but no they don't have a vacuum in them. They're called vacuum tubes because the early tube makers attempted to get the oxygen out of them. Edison himself failed at that so he started experimenting with inert gasses to displace the oxygen. Todays tubes have a partial vacuum with an inert gas to replace the air.

Sorry Bob, but they really do have a vacuum in there. As always, it is a partial vacuum - there's no such thing as an absolute vacuum, but there is no inert gas in standard audio vacuum tubes. Here's a quote from one tube page:

Figure 3 (Inside a miniature tube) shows a typical modern vacuum tube. It is a glass bulb with wires passing through its bottom, and connecting to the various electrodes inside. Before the bulb is sealed, a powerful vacuum pump sucks all the air and gases out. This requires special pumps which can make very "hard" vacuums. To make a good tube, the pump must make a vacuum with no more than a millionth of the air pressure at sea level (one microTorr, in official technical jargon). The "harder" the vacuum, the better the tube will work and the longer it will last. Making an extremely hard vacuum in a tube is a lengthy process, so most modern tubes compromise at a level of vacuum that is adequate for the tube's application.

http://www.vacuumtubes.net/How_Vacuum_Tubes_Work.htm


Do you have a citation for the inert gas thing? Voltage regulator tubes and thyratrons contain gas, but not guitar amp tubes.

For a discussion of "blue glow", see here:

http://www.kcanostubes.com/content/news ... ticleID=10


A Google search will get you many other pages on this subject.

[Bob-I]

Sorry Bob, but they really do have a vacuum in there. As always, it is a partial vacuum - there's no such thing as an absolute vacuum, but there is no inert gas in standard audio vacuum tubes. Here's a quote from one tube page:

Ok, it's not worth fighting over. This is what I learned with I got my engineering degree many years ago, the one that had to be chizzled in stone because the printing press hadn't been invented yet.

Let's get back to the orignal question... no that glow is not a problem.

[rfgordon]

While I appreciate animated banter as much as the next guy, I'd still like to know the answer to my last question: How come nobody (at least I've never seen it done) puts a bypass cap on the cathode resistor of the PI?

I thank you all very much for the other answers. Y'all are, as always, extremely knowledgeable and helpful.

[Bob-I]

While I appreciate animated banter as much as the next guy, I'd still like to know the answer to my last question: How come nobody (at least I've never seen it done) puts a bypass cap on the cathode resistor of the PI?

Yea... we got away from the topic, sorry.

My impression is that the PI is not used to add gain. Yes there's added gain but in general it's job is not to add gain but to divide the wave into the 2 halves. I've never seen a design with a bypass cap but I don't see why it couldn't be used.

Theres a great page on the PI on Aiken's site. Check it out at http://www.aikenamps.com/LongTailPair.htm

[VacuumVoodoo]

You're talking LTP, right? If so, look at the right hand triode as common/grounded grid amplifier i.e. its input is the cathode. Put a cap from the cathode to ground and you effectively shunt it to gnd. The result will be a straight forward common cathode gain stage on the left triode. The right side triode will just idle away sleeping peacefully.

Bob-I: LTP has gain, between 26 and 30db of it. Split load or Cencertina PI has 0db gain by virtue of design.

[rfgordon]

Thanks very much for clearing that up for me. As always, y'all are great.